1. Field of the Invention
The present invention relates to an improved apparatus for coating a surface of a substrate with a material (such as diamond film) using a plasma generated by a microwave or UHF power source. In particular, the present invention relates to an apparatus which generates a relatively wide plasma adjacent to the surface of the substrate being coated to provide a uniform coating across the surface of the substrate.
2. Prior Art
Providing a thin non-reactive film of a material on a substrate to improve the properties of the substrate has always been an important part of scientific and industrial research. The demand for coated substrates leads to the interest in the design and construction of production apparatus to accomplish these technological advancements.
Diamond, having unique mechanical, optical and electrical properties, is one of the most valuable scientific and technological materials. In 1955, Bundy and co-workers succeeded in the reproducible synthesis of diamond (F. P. Bundy, H. T. Hall, H. M. Strong, and R. H. Wentoff, Jr., "Man-made diamond", Nature, 176, 51 (1955)) with a molten transition metal solvent-catalyst at pressures where diamond is the thermodynamically stable phase of carbon.
Diamond growth at low pressures where graphite is the stable carbon phase can be traced back to W. G. Eversole (W. G. Eversole, U.S. Pat. Nos. 3,030,187 and 3,030,188)), Angust et al. (J. C. Angus, H. A. Will, and W. S. Stanko, J. Appl. Phys. 39, 2915 (1968)); and Deryaguin et al. (B. V. Deryaguin, D. V. Fedoseev, V. M. Lukyanovich, B. V. Spitsyn, V. A. Ryabov, and A. V. Lavrentyev, J. Cryst. Growth 2, 380 (1968)), but the low growth rate (less than 0.1 micrometer per hour) could not be of practical commercial importance and thus prevented worldwide interest at that time.
A breakthrough in the synthesis of diamond at low pressures came in the late 1970's and early 1980's, when a group of Soviet researchers (B. V. Spitsyn, L. L. Bouilov and B. V. Deryagin, J. Cryst. Growth 52, 219 (1981)) and Japanese researchers (S. Matsumoto, Y. Sato, M. Kamo, N. Setaka, Jpn. J. Appl. Phys. 21, part 2, 183 (1982)) published a series of research papers on diamond film growth at a higher growth rate (several micrometers per hour) from hydrocarbon-hydrogen gas mixtures. Since then, various techniques have been developed for diamond film growth at low pressures. These techniques can be divided into five major categories: (1) thermally activated or hot filament activated chemical vapor deposition (CVD) (S. Matsumoto, Y. Sato, M. Kamo, N. Setaka, Jpn. J. Appl. Phys. 21, part 2, L183 (1982); S. Matsumoto, Y. Sato, M. Tsutsumi and N. Setaka, J. Mater. Sci., 17, 3106 (1982)); (2) high frequency plasma enhanced CVD (M. Kamo, Y. Sato, S. Matsumoto and N. Setaka, J. Cryst. Growth 62, 642 (1983); S. Matsumoto and Y. Matsui, J. Mater. Sci., 18, 1785 (1983); S. Matsumoto, J. Mater. Sci. Lett, 4, 600 (1985); S. Matsumoto, M. Hino and T. Kobayashi, Appl. Phys. Lett., 51, 737 (1987)); (3) direct current discharge enhanced CVD (K. Suzuki, A. Sawabe, H. Yasuda, and T. Inuzuka, Appl. Phys. Lett., 50, 728 (1987)); (4) combustion flame (Y. Hirose and M. Mitsuizumi, New Diamond, 4, 34 (1988)); and (5) other and hybrid techniques. All of these techniques are based on the generation of atomic hydrogen near the thin film growing surface. A very common method of synthesis is microwave plasma assisted CVD. This method has shown excellent potential for growing high quality diamond films and variations of this technique are now in common use in many laboratories, but has not reached the stage of commercialization.
There is a need for the development of microwave plasma reactors for the deposition of diamond and diamond thin film coating on other materials. Since there are no electrodes present in the microwave plasma, the problem of metallic contamination in the process of diamond deposition does not exist. Compared to the erosion of filaments in hot filament reactors, erosion of electrodes in direct current reactors and nozzle erosion in combustion flame reactors, microwave plasma diamond film deposition is a cleaner process. It is also easier to control and optimize the deposition process which makes microwave plasma reactors the most promising technique for stably growing pure, uniform, and high quality diamond films (C. V. Deshpandey and R. F. Bunshah, J. Vac. Sci. Technol. A7, 2294 (1989); W. Zhu, B. Stoner, B. Williams, and J. T. Glass, Proc. IEEE 79, no. 5, 621 (1991)).
Diamond film deposition using microwave plasma has been achieved by several different apparatus. The first was reported by Kamo et al. (M. Kamo, Y. Sato, S. Matsumoto and N. Setaka, J. Cryst. Growth 62, 642 (1983)). A silica tube 40 millimeter in diameter passes through the sleeves attached to the waveguide. The tube serves as the deposition chamber. Microwave energy (2.45 GHz, 9.15 MHz) generated by the magnetron is transmitted to the chamber through a set of waveguides, a power monitor and a tuner. The position of the plasma was adjusted into the center of the deposition chamber, where the substrate was held by an alumina basket. Deposition was performed by passing a mixture of hydrogen and methane to the chamber and then applying microwave power to induce a microwave glow discharge.
A magneto-microwave plasma reactor for diamond growth was reported by Kawarada et al (H. Kawarada, K. S. Mar and A. Hiraki, Jpn. J. Appl. Phys. 26, L1032 (1987)). It is composed of a microwave generator, a waveguide, Helmholtz-type magnetic coil, a round waveguide (a discharge area) with TE11 mode, and a reaction chamber. The inner diameter of the round waveguide is 160 millimeter. The magnetic field distribution is intended to control the deposition under a high density plasma. The plasma density is the highest around the position where the magnetic field corresponds to the electron cyclotron resonance conditions (875 Gauss in this case). The position can be moved by the intensity of applied magnetic field and be set around the substrate. Observed diamond particles and films have been obtained on whole substrates as large as 30 millimeters in diameter. The pressure where the high quality diamond has been formed is around 10 Torr. Diamond films were deposited at pressures from 4 to 50 Torr.
Bachmann et al. (P K. Bachmann, W. Drawl, D. Knight, R. Weimer, and R. F. Messier, in Extended Abstracts Diamond and Diamond-like Materials Synthesis, edited by G. Johnson, A. Badzian, and M. Geis, Materials Research Society, Pittsburgh, Pa., 1988, p. 99) reported a bell jar microwave plasma reactor for diamond film growth. The cavity allows the insertion of a 1 inch graphite disk as a substrate holder for experiments with microwave/plasma heated samples. The interior of the bell jar reactor can be replaced by a 3 inch flat spiral resistance heater that can be mounted through the bottom of the cavity. Deposition experiments using plasma heated single crystal silicon substrates resulted in diamond coating over sample diameters of 25 millimeters. Coatings of more than 65 millimeter (2.75 inch) diameter were deposited on silicon, using the separate substrate heater. Pressure variations between 40 and 70 Torr were found to have little effect on the diamond growth rate in a bell jar reactor. The variation of the total flow between 200 sccm and 600 sccm (while keeping the methane concentration constant at 0.5%) did not affect the growth rate. The plasma formed in the reactor has ball shape.
A microwave disk reactor for diamond film growth was reported by the present inventors et al (J. Zhang, B. Huang, D. K. Reinhard and J. Asmussen, J. Vac. Sci. Technol. A 8, 2124 (1990)) and tested in a commercial setting. The disk reactor consists of the cylindrical side walls which form the outer conducting shell of the cavity applicator. The water-cooled sliding short, the cavity bottom surface, and the water-cooled base-plate, along with the cavity side walls, form the cylindrical excitation cavity. Input gas flows into the quartz chamber via the input gas channel inside the base-plate. A 92.5 millimeter inside diameter quartz disk confines the working gas to the lower section of the cavity applicator where the microwave fields produce a hemisphere shaped plasma adjacent to the substrate. Microwave power is coupled into the cavity through the side-feed coaxial power input probe. A discharge is ignited in the disk shaped zone by exciting the cavity in a single plasma loaded resonant mode. The substrate temperature is measured through the screened top window using an optical pyrometer.
To successfully commercialize diamond synthesis at low pressures, diamond growth at high rates and low deposition temperatures on large area substrates is desirable. Each of the microwave apparatuses described above has its advantages and disadvantages for diamond film deposition. What their disadvantages have in common is that they can not be easily scaled up for large area diamond film deposition. The reactor reported by Kamo et al has two disadvantages, first, the substrate size is limited by the inside diameter of the silica tube, which is 40 millimeters, and the system is not easily scalable for diamond film growth on a large surface since the diameter of the silica tube is limited by the size of the rectangular waveguide; second, the plasma generated inside the silica tube is very close to the inside walls, under the conditions suitable for diamond deposition, erosion of the silica walls and hence contamination of the diamond film are likely. The reactor reported by Kawarada et al uses a narrow, electron cyclotron resonance ring to generate the high density plasma ring. Non-uniformity in the deposited film can be expected as an inherent result of the non-uniform excitation of the plasma, especially for diamond deposition over a larger surface. The reported surface area for diamond growth was 30 millimeters in diameter. It has a narrow pressure region for diamond film growth, namely from 4 to 50 Torr. At pressures above 50 Torr, the discharge area becomes unstable and below 4 Torr, the products contain graphite or SiC phase and in extreme cases the products are only these phases. The reactor reported by Bachmann et al. uses a plasma ball and the substrate is located near the lower pole of the plasma ball. The reactive species distribution over the substrate surface is inherently non-uniform. This is especially true when the substrate surface extends further away from the lower pole of the plasma ball. The reported coating surface area is 25 millimeters in diameter with the plasma. A separate heater is needed to coat a surface of 65 millimeters in diameter. The location of the substrate and substrate holder are fixed in order to generate the ball shaped plasma. External tuning is needed in order to minimize the reflected power from the reactor since there is only one internal adjustment (i.e. the antenna) available for microwave coupling.
The microwave disk reactor reported by Asmussen et al uses a hemisphere shaped plasma to deposit diamond film on a substrate. The reactive species covering a surface of 40 millimeters in diameter is uniform. But in this reactor, the power input comes from the side of the cavity walls and produces an inherent non-uniform electromagnetic field near the excitation probe. This near field effect gets stronger as the input power and quartz disk inside diameter are increased, creating a non-uniform and unstable plasma. Hence this reactor cannot be used to uniformly deposit diamond on surface areas larger than 50 millimeters in diameter when operating at pressures above 5 Torr.
U.S. Pat. No. 4,906,900 to Asmussen describes an apparatus wherein a probe is aligned parallel to, but offset from the longitudinal axis. The purpose of this invention was to provide a long narrow apparatus for retrofitting existing vacuum sources. There is a suggestion that the apparatus could be used for diamond thin films; however, the device does not have sufficient cross-sectional areas in the chamber for commercial purposes.
U.S. Pat. No. 4,792,772 to Asmussen describes a more conventional commercially available apparatus. The problem is that this apparatus does not provide a completely uniform plasma at the high pressures (i.e. 20 to 100 Torr) desirable for high growth rates because of the position of the probe. U.S. Pat. No. 4,943,345 to Asmussen and Reinhard shows an apparatus wherein a nozzle is provided to direct the flow of the excited species from the plasma. U.S. Pat. No. 4,691,662 to Roppel, Asmussen and Reinhard describes a dual plasma device. U.S. Pat. No. 4,630,566 to Asmussen and Reinhard; U.S. Pat. No. 4,727,293 to Asmussen, Reinhard and Dahimene, and U.S. Pat. No. 4,585,668 to Asmussen show various plasma generating devices. All of these patents describe apparatus where the probe enters the cavity from the side perpendicular to the axis of the cavity.